Pwm illumination control circuit with low visual noise for driving led

ABSTRACT

A pulse width modulation (PWM) illumination control circuit with low visual noise for driving a light-emitting diode (LED) is provided. An illumination control pulse-generating unit is used to generate an illumination control pulse signal according to an illumination-adjusting signal. The duty cycle or frequency of the illumination control pulse signal varies within a predetermined scope for controlling a DC/DC converter to drive the light-emitting diode so that overall visual noise level of the PWM illumination control circuit is improved.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/708,212 filed on Feb. 17, 2003 which claims the priority benefitof Taiwan application serial no. 92134517, filed on Dec. 8, 2003. Theentirety of each of the above-mentioned patent applications is herebyincorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an illumination control circuit. Moreparticularly, the present invention relates to a pulse width modulation(PWM) illumination control circuit with low visual noise for driving alight-emitting diode (LED).

2. Description of the Related Art

In recent years, conventional cathode ray tubes (CRT) are graduallybeing replaced by liquid crystal displays (LCD) due to big improvementsin the semiconductor manufacturing techniques. LCD has many advantagesover CRT including lower power consumption, a lighter weight, a higherresolution, higher degree of color saturation and a longer service life.For these advantages, LCD is being widely used in many electronicproducts including digital cameras, notebook computers, desktopmonitors, mobile phones, personal digital assistants (PDA), cartelevision, global positioning systems (GPS), palm-top game player,electronic translators and even digital watches and so on.

In general, a liquid crystal display uses an array of light-emittingdiodes (LED) driven by a simple driving circuit to serve as the lightsource. However, due to the special properties of an LED, brightness ofthe LED is not linearly related to the driving current. Furthermore,color of the LED may also vary according to the driving current. Hence,for a liquid crystal display that uses LED as a back light orillumination system, difficulties are often encountered when theillumination is varied by directly adjusting the driving current.

To avoid the difficulties of illumination adjustment through anamplitude variation of the driving current, a constant amplitude drivingcurrent is used with illumination adjustment achieved through a pulsewidth modulation (PWM) of the driving current. Ultimately, the LED isable to produce consistent emitting efficiency within a broad range.

FIG. 1 is a block diagram of a conventional pulse width modulationillumination control circuit. FIG. 2 is a diagram showing therelationships between illumination control pulse signals andlight-emitting diode driving current signals for the circuit in FIG. 1.In FIG. 1, an illumination control pulse signal Con that set theillumination of the light-emitting diode is sent to a DC/DC converter110 to produce a light-emitting diode driving current signal Id fordriving a light-emitting diode. The waveform diagrams (a), (b) and (c)in FIG. 2 represent three different pulse width settings of thelight-emitting diode driving current signals Id. For example, thelight-emitting diode is at full illumination (100%) in FIG. 2(a), at 20%of the full illumination in FIG. 2(b) and at 50% of the fullillumination in FIG. 2(c).

To prevent any perceived flickering in the light-emitting diode by thehuman eyes, the frequency of the illumination control pulse signal Concannot be too low, typically above 200 Hz. In other words, theillumination control pulse signal Con must operate at a sufficientlyhigh frequency so that the human eyes can retain a visual image and yetperceive a steady change of illumination without flickering. Obviously,these control signals may be implemented using a simple switchingcircuit that controls the on/off states of the entire DC/DC converter.

Because the frequency and duty cycle of the aforementioned illuminationcontrol pulse signal Con is set to be fixed according to the requiredillumination, interference with the vertical, horizontal scanningsignals may occur when used as back light in a liquid crystal display.The difference in frequency between the back light and the video signalsoften leads to the so-called ‘fanning effect’, a watery wave pattern ofthe image on a display screen. In addition, the switching on or off ofthe DC/DC converter also leads to a significant loading on the powersupply that provides power to the DC/DC converter. In other words, aripple waveform synchronized with the illumination control pulse signalCon is also produced in the power supply. Once again, the ripplewaveform may affect the video display signals leading to a flickeringscreen.

To prevent interference between the illumination control pulse signalCon and the vertical, horizontal scanning signal due to the frequencydifference, the illumination control pulse signal Con and the horizontalscanning signals are synchronized to a frequency an integral multiple ofeach other. However, this arrangement will increase the production cost.To reduce the ripple waveform in the power supply, the frequency of theillumination control pulse signal Con can be increased. Yet, increasingthe frequency of pulse signal Con leads to higher power consumption.With the demand of a larger display screen and a lesser visual noise,fabricating a light-emitting diode illuminated liquid crystal displaywith low noise and broad adjustable range of illumination isincreasingly difficult.

SUMMARY OF THE INVENTION

Accordingly, one objective of the present invention is to provide apulse width modulation (PWM) illumination control circuit with lowvisual noise for driving a light-emitting diode (LED). By varying theduty cycle or frequency of an illumination control pulse signal andmaintaining an average duty cycle and frequency, visual noiseinterference due to pulse width modulation is reduced.

To achieve these and other advantages and in accordance with the purposeof the invention, as embodied and broadly described herein, theinvention provides a low visual noise pulse width modulationillumination control circuit for controlling the illumination oflight-emitting diodes inside a liquid crystal display. The low visualnoise pulse width modulation illumination control circuit comprises anillumination control pulse generating unit and a DC/DC converter. Theillumination control pulse-generating unit receives anillumination-adjusting signal. According to the illumination-adjustingsignal, the illumination control pulse-generating unit generates anillumination control pulse signal having a duty cycle set to vary withina predetermined range. The DC/DC converter is coupled to theillumination control pulse-generating unit so that the illuminationcontrol pulse-generating unit can drive the light-emitting diodesaccording to the illumination control pulse signal.

In one embodiment of the invention, the illumination controlpulse-generating unit of the low visual noise PWM illumination controlcircuit further comprises a noise generator, an analogue adder and acomparator. The noise generator generates a noise signal. The analogueadder is coupled to the noise generator for receiving theillumination-adjusting signal and the noise signal to produce a noisesignal loaded illumination-adjusting signal. The comparator is coupledto the analogue adder for comparing the noise signal loadedillumination-adjusting signal with a triangular wave and producing theillumination control pulse signal.

In one embodiment of the invention, the noise signal level produced bythe low visual noise PWM illumination control circuit can be adjusted.

The present invention also provides an alternative low visual noisepulse width modulation illumination control circuit for controlling theillumination of light-emitting diodes inside a liquid crystal display.The low visual noise pulse width modulation illumination control circuitcomprises an illumination control pulse generating unit and a DC/DCconverter. The illumination control pulse-generating unit receives anillumination-adjusting signal. According to the illumination-adjustingsignal, the illumination control pulse-generating unit generates anillumination control pulse signal having a frequency set to vary withina predetermined range. The DC/DC converter is coupled to theillumination control pulse-generating unit so that the illuminationcontrol pulse-generating unit can drive the light-emitting diodesaccording to the illumination control pulse signal.

In one embodiment of the invention, the operations carried out by theillumination control pulse-generating unit of the low visual noise PWMillumination control circuit are implemented using a microprocessor.

In one embodiment of the invention, the phase of the illuminationcontrol pulse signal produced by the low visual noise PWM illuminationcontrol circuit also varies within a predetermined range.

Accordingly, the present invention provides a low visual noise PWMillumination control circuit for driving light-emitting diodes such thatvisual noise interference due to pulse width modulation is reduced byvarying the duty cycle or frequency of an illumination control pulsesignal and maintaining an average duty cycle and frequency.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The following drawings illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

FIG. 1 is a block diagram of a conventional pulse width modulationillumination control circuit.

FIG. 2 is a diagram showing the relationships between illuminationcontrol pulse signals and light-emitting diode driving current signalsfor the circuit in FIG. 1.

FIG. 3 is a block diagram of a light-emitting diode low visual noise PWMillumination control circuit according to one preferred embodiment ofthe present invention.

FIG. 4 is a circuit diagram of an illumination control pulse-generatingunit according to the preferred embodiment of the present invention.

FIG. 5 is a diagram showing the waveform of the illumination controlpulse signal produced by the illumination control pulse-generating unitshown in FIG. 4.

FIG. 6 is a flow chart showing the steps for operating the illuminationcontrol pulse-generating unit according to the preferred embodiment ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

FIG. 3 is a block diagram of a light-emitting diode low visual noise PWMillumination control circuit according to one preferred embodiment ofthe present invention. The low visual noise pulse width modulation (PWM)illumination control circuit 300 in FIG. 3 is adapted to control theillumination level of light-emitting diodes (not shown) inside a liquidcrystal display. The low visual noise PWM illumination control circuit300 comprises an illumination control pulse-generating unit 310 and aDC/DC converter 320. The illumination control pulse-generating unit 310is used for receiving an illumination-adjusting signal Ref. According tothe illumination-adjusting signal Ref, the illumination controlpulse-generating unit 310 generates an illumination control pulse signalCon. To improve the visual noise interference of pulse width modulation,the duty cycle or frequency of the illumination control pulse signal Conis permitted to vary within a predetermined range. Hence, differentialfrequency interference between a fixed illumination control pulse signalCon and the vertical/horizontal scanning signal leading to the so-called‘fanning effect’ with wavy lines on the display screen is prevented. Inaddition, the DC/DC converter 320 drives the light-emitting diodesaccording to the illumination control pulse signal Con generated by theillumination control pulse-generating unit 310.

FIG. 4 is a circuit diagram of an illumination control pulse-generatingunit according to the preferred embodiment of the present invention. Asshown in FIG. 4, the illumination control pulse-generating unit 400comprises a noise generator 410, an analogue adder 420 and a comparator430. The noise generator 410 further comprises a resistor 411 and anamplifier 421 electrically connected together and the analogue adder 420further comprises a plurality of resistors 422, 423, 425 and anamplifier 421 electrically connected together. The noise generator 410outputs a noise signal Nos after the amplifier 412 inside the noisegenerator 410 amplifies the thermal noise produced by the resistor 411.The noise signal Nos is transmitted to the analogue adder circuit 420such that the noise signal Nos and an illumination-adjusting signal Reforiginally set to control the output duty cycle of the DC voltage aresummed together to produce a noise signal loaded illumination-adjustingsignal Ref. The resistor 422 is a variable resistor so that the level ofthe noise signal Nos loaded on the illumination-adjusting signal Ref canbe adjusted. The noise signal loaded illumination-adjusting signal Refis transmitted to the comparator 430 where the signal is compared with atriangular wave Tri to produce an illumination control pulse signal Conhaving a duty cycle that varies within the acceptable noise signal levelas shown in FIG. 5.

As shown in FIG. 5, although the duty cycle of the illumination controlpulse signal Con varies on each transient moment of each cycle, theaverage power of the noise is zero. Hence, the average duty cycle of theentire circuit after adding the noise is identical to one without addingany noise. In other words, the illumination of the light-emitting diodesafter adding noise to the circuit is identical to the illuminationwithout adding any noise to the circuit.

FIG. 6 is a flow chart showing the steps for operating the illuminationcontrol pulse-generating unit according to the present invention. Whenthe illumination control pulse-generating unit 310 as shown in FIG. 3 isimplemented using a microprocessor, the steps in FIG. 6 can be carriedout to produce an illumination control pulse signal Con with a variablefrequency so that visual noise interference due to pulse widthmodulation is reduced.

Assuming that the illumination control pulse signal Con in FIG. 3 has afrequency F=1/T, where T is the cycle of the illumination control pulsesignal Con, n illumination control pulse signals Con with differentcycle time such as T0, T1, T2, . . . , Tn−1 such that (T0+T1+T2+ . . .+Tn−1)/n=T can be designed. Furthermore, the n illumination controlpulse signals Con with different cycle time can be permuted to form aqueue before turning each signal out sequentially. For example, ifsequence 0 is {T0, T1, T2, . . . , Tn−1}, sequence 1 is {T0, T2, . . .,} and so on, the n illumination control pulse signals Con withdifferent cycle time may be arranged to form a list of K differentnon-repeating sequence including sequence 0, sequence 1, sequence 2, . .. , sequence K−1. Thereafter, the steps depicted in FIG. 6 can beexecuted using the microprocessor so that illumination control pulsesignals Con each having a different frequency are sequentially output.The operating steps of a digitally operated illumination controlpulse-generating unit with a low visual noise level are explained asfollows.

In step S610, variables I, J are set to 0. Thereafter, in step S620, theI^(th) illumination control pulse signal cycle in sequence J and thereceived illumination-adjusting signal are combined to produce anillumination control pulse signal. In step S630, a 1 is added to thevariable I in preparation for retrieving the next illumination controlpulse signal cycle in sequence J. In step S640, the value of I ischecked to determine whether it is equal to n. When the value of I isnot equal to n, the operation returns to step S620. However, if thevalue of I is equal to n, step S650 is executed to reset I to 0 and add1 to the value of J in preparation for retrieving the first illuminationcontrol pulse signal cycle of the next sequence. Thereafter, step S660is executed to determine whether the value of J is equal to K. When thevalue of J is not equal to K, the operation returns to step S620. On theother hand, if the value of J is equal to K, step S670 is executed toreset the value of J to 0 and return the operation to step S620.

The steps carried out in aforementioned description assumes theexistence of K sequences. However, anyone familiar with the techniquemay understand that the operation is greatly simplified when K is 1. Inaddition, the phase of the illumination control pulse signal generatedin step S620 can be set to vary within a predetermined range so that anillumination control pulse signal with a wider frequency range isproduced.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A pulse width modulation illumination control circuit for controllingthe illumination of light-emitting diodes inside a liquid crystaldisplay, comprising: an illumination control pulse-generating unit, forreceiving an illumination-adjusting signal and generating anillumination control pulse signal according to theillumination-adjusting signal, wherein a frequency of the illuminationcontrol pulse signal varies with time within a predetermined range; anda DC/DC converter, coupled to the illumination control pulse-generatingunit for driving the light-emitting diodes according to the illuminationcontrol pulse signal.
 2. The control circuit of claim 1, wherein theillumination control pulse-generating unit is implemented using amicroprocessor.
 3. A pulse width modulation illumination control circuitfor controlling the illumination of light-emitting diodes inside aliquid crystal display, comprising: an illumination controlpulse-generating unit, for receiving an illumination-adjusting signaland generating an illumination control pulse signal according to theillumination-adjusting signal, wherein a phase shift of the illuminationcontrol pulse signal varies with time within a predetermined range; anda DC/DC converter, coupled to the illumination control pulse-generatingunit for driving the light-emitting diodes according to the illuminationcontrol pulse signal.
 4. An illumination control pulse generating devicefor receiving an illumination adjusting signal, the illumination controlpulse generating device comprising: a random signal generating unit, forgenerating a random signal; a combine unit, coupled to the random signalgenerating unit for combining the random signal with the illuminationadjusting signal to form a random signal combining illuminationadjusting signal; and a comparator, coupled to the combine unit forcomparing the random signal combining illumination adjusting signal witha comparing signal to produce a illumination control pulse signal,wherein a frequency of the illumination control pulse signal varies withtime within a predetermined range.
 5. The illumination control pulsegenerating device of claim 4, wherein the random signal generating unitis a noise generator.
 6. The illumination control pulse generatingdevice of claim 4, wherein the combine unit is an analogue adder.
 7. Theillumination control pulse generating device of claim 4, wherein thecomparing signal is a triangular wave.